In addition to the measurement of line widths or further variables that characterize individual structures, the measurement of absolute and relative positional accuracies of the respectively projected structure patterns also plays an important part in semiconductor fabrication. In particular, a high degree of a relative positional accuracy of a structure pattern with respect to a previously imaged structure pattern ensures the function of the relevant circuit and thus a high yield.
Relative positional accuracies are usually determined by means of measurement marks inserted into the structure patterns specifically for this purpose. In principle, this involves measuring the offset of a second measurement mark part, which is formed in an upper layer on the wafer, with respect to a first measurement mark part formed in an underlying layer. The measurement is carried out, e.g., in a measuring microscope (overlay measuring device) operating under optical light (also ultraviolet light). A detailed measurement in a scanning electron microscope (SEM) is equally possible.
Structures suitable for such a measurement are the so-called box-in-box structures, for example. A measurement mark part comprising four bars combined to form a quadrangle is formed in each of the two layers. The quadrangle of one layer has a different diameter from the quadrangle of the other layer, so that the two quadrangles are nested in one another about a common midpoint in the case of an ideal imaging. The bars, i.e., the structure elements of the underlying layer, can be detected by the measuring device either because the upper layer has a degree of transparency, or because the depression caused by the bars or the ridge is reflected in the topography of the overlying layer.
If an imaging error is present, then the two measurement mark parts are imaged with a relative offset with respect to one another, it being possible to read this offset directly with the aid of the measuring device with the altered bar position within the wafer plane (XY plane). The imaging error may be caused for example by lens aberrations, by vibrations or by shocks while carrying out the projection step, but also by problems during postprocessing—for example etching, polishing deposition processes, etc. The last-mentioned problems are imaging errors in the broader sense since the processes mentioned only lead to an apparent displacement of the measurement mark parts or structures within the wafer plane. In the case of the so-called “snowdrift effect,” increased deposition thicknesses are obtained during the deposition in a preferred direction around, e.g., elevated structures, the form of said deposition thicknesses being reminiscent of the “snowdrifts” mentioned. If the position of the measurement mark part is then determined solely from the surface topography, then an apparently displaced measurement mark is present.
The polishing process may give rise to an edge removal and thus to a reduction of contrast at the edges of the bars. In the case of elevated structures of a measurement mark part, that edge of a bar structure which faces the respective polishing direction is affected by the degradation to a greater extent than the edge remote from the polishing direction.
Precisely those apparent imaging errors caused by postprocessing can be detected, and thus taken into account, only with very great difficulty by optical measuring devices for determining the positional accuracy.
A further problem arises from the fact that the ongoing reduction of widths with which structures in a semiconductor circuit are to be fabricated in order to increase the integration density is advancing faster, on account of progress in the area of lithographic imaging, than is possible through an improved resolution of the optical measuring devices in the area of metrology in the context of detecting structures with corresponding widths. Tolerances with regard to the positional accuracy of 10 nm are prescribed in the case of the 110 nm technology generation. However, a measurement uncertainty of 1–3 nm already has to be taken into account on the part of the measuring devices. A transition to reduced tolerance limits of 8 nm or less is already expected in the near future, particularly if the transition to the 90 nm or 70 nm technology generation is accomplished. The relative proportion made up by the measurement uncertainty in the tolerance budget accordingly increases, which is not acceptable in the long term.
As an alternative, it is possible to carry out measurements using scanning electron microscopes; for a fast inline measurement within production, however, this alternative is not taken into consideration in view of the high outlay and the associated prolonged production time.